PROJECT 3 - SUMMARY/ABSTRACT Alzheimer's disease (AD) is a complex neurodegenerative disorder caused by interactions among multiple genetic and environmental factors. Apolipoprotein (apo) E4 increases the risk and lowers the age of onset for AD in a gene dose-dependent manner. Remarkably, the lifetime risk estimate of developing AD for individuals with 2 copies of the apoE4 allele (~2% of the population) is ~70% by the age of 85. By comparison, the lifetime AD risk estimate for individuals with two copies of the apoE3 allele is ~10% by the age of 85. Although many of the apoE4 homozygotes develop early-onset AD (EOAD) or late-onset AD (LOAD), some of them (~20%) stay asymptomatic over age 85. Understanding the susceptibility of the former group and the resistance of the latter group to AD might allow for the development of strategies to prevent or delay AD in people at risk. Like AD, frontotemporal dementia (FTD) is also a multifactorial and heterogeneous neurodegenerative disorder. About 20-50% of FTD cases are inherited, and heterozygous mutations in the progranulin (PGRN) gene are one of the most common causes of the inherited forms of FTD. However, homozygous PGRN mutations unexpectedly cause neuronal ceroid lipofuscinosis (NCL) rather than FTD. Understanding how the same mutation within a single PGRN gene causes different phenotypes depending on gene dose is crucial for unraveling the pathogenesis of both FTD and NCL and for their therapeutic developments. Because neurons cannot be obtained directly from patients, induced pluripotent stem cells (iPSCs) derived from AD or FTD patients hold great promise as in vitro models for studying disease pathogenesis in human neurons. This proposal aims to capitalize on this promise by building on our previous efforts to generate a well- characterized repository of human iPSC (hiPSC) lines from AD and FTD patients and, by using them, to study disease phenotypes, reveal novel mechanisms, and identify new therapeutic targets. The goals of this project are to address the following two questions: (1) why are many apoE4 homozygotes vulnerable to AD pathogenesis at young ages when others (~20%) can stay asymptomatic by age 85 and over? (2) How do heterozygous and homozygous mutations within a single PGRN gene cause different clinical phenotypes, i.e., FTD and NCL, respectively? To answer these questions, we will compare the phenotypic differences (e.g., A production/secretion and tau phosphorylation/fragmentation) among neurons derived from apoE4/4-hiPSC lines from EOAD patients, LOAD patients, and asymptomatic controls at different ages (Aim 1); determine the transcriptomic and proteomic differences among neurons derived from apoE4/4-hiPSC lines from EOAD patients, LOAD patients, and asymptomatic controls at different ages (Aim 2); and explore the underlying mechanisms by which heterozygous PGRN mutations cause FTD but homozygous PGRN mutations cause NCL (Aim 3). These studies should significantly accelerate AD and FTD research and related therapeutic development by enhancing our understanding of disease pathogenesis at the molecular and cellular levels.